the.com/theirs
The possessive pronoun that owns everything without ever wanting an apostrophe.
means Belonging to them; the form of their that stands alone without a noun following it.
from From Old Norse theirra, muscled into English by Vikings, replacing the native hira around the 1300s.
No apostropheThere's is a verb; theirs is never that.
Viking importBorrowed from Norse, not native Old English.
Absolute pronounStands alone: that book is theirs.